Geoscience Reference
In-Depth Information
exposure experiments in rats, developmental neurotoxic effects can be induced,
such as aberrations in spontaneous behavior, learning, and memory function
(Eriksson et al.
2002
). HBCDs can also alter the normal uptake of neurotrans-
mitters in rat brains (Mariussen and Fonnum
2003
).
4.3 Petroleum Hydrocarbons and Fuel Additives
Petroleum hydrocarbons (PHs) constitute a group of compounds characterized by
complex mixtures of hydrocarbons. Overall, hundreds to thousands of individual
compounds can be found in the different mixtures, although information about
their physical and chemical properties is available only for approximately 250 of
these compounds, and substantial toxicological data exist for just a small fraction
(*10 %) of the identified substances (Vorhees et al.
1999
). Often these com-
pounds are referred to in the literature as light nonaqueous-phase liquids
(LNAPLs), when they exist as a separate phase. In terms of volumes of contam-
inants released to the environment, the contamination of land surface, partially
saturated zone, and groundwater by PHs generally is one of the most serious. This
is due to the staggering amounts of PHs used mainly as energy sources for elec-
tricity, transportation, and heating around the world. Leaking underground and
aboveground storage tanks, improper disposal of petroleum wastes, and accidental
spills are major routes of soil and groundwater contamination by petroleum
products (Nadim et al.
2000
).
Upon release to the environment, the bulk phase migrates downward by gravity.
As the NAPL moves through the partially saturated zone, a fraction of the PH is
retained by capillary forces as residual globules in the soil pores, thereby depleting
the contiguous PH mass until movement ceases. If sufficient PH is released, it will
migrate until it encounters a physical barrier (e.g., low-permeability stratum) or is
affected by buoyancy forces near the water table. Additionally, PH vapors migrate
in the porous matrix creating a larger impact zone. Once the capillary fringe is
reached, the PH may move laterally as a continuous, free-phase layer along the
upper boundary of the water-saturated zone, due to gravity and capillary forces. On
contact with water in the saturated or partially saturated zone, dissolution of
compounds from the PH mixture begins. A schematic description of PH distri-
bution patterns in the subsurface is given in Fig.
4.7
.
There are different approaches to estimating the toxicity of various PHs. One
method is to examine the known individual compounds in each PH fraction, based
on the data collected for a limited number of compounds and assuming that the
known materials are representative of the entire mixture. A second method is to
divide the mixture into several fractions that contain substances with similar
chemical and physical properties, which therefore are considered to have com-
parable toxicity. A third approach is to consider the entire mixture. The actual
content of each mixture depends mainly on the origin of the PH and the distillate
fractions.
